[llvm-dev] InlineSpiller - hoists leave virtual registers without live intervals

[via llvm-dev]

From: Wei Mi <wmi at google.com> 
> I havn't noticed before that TII.storeRegToStackSlot is allowed to create new virtual registers. I am surprised to know it is allowed to do that. I am mostly working on x86 and TII.storeRegToStackSlot for x86 doesn't create new register. As you said, if TII.storeRegToStackSlot is allowed to create new virtual registers, that could be a problem. It is not exposed maybe because of some later pass cover the problem -- maybe RegScavenger? 

> Are you aware of any other architecture other than AMDGPU on which TII.storeRegToStackSlot creates new virtual registers? Could you explain in which case new virtual registers will be created? 

It is quite possible it's a largely forgotten feature from before RegScavenger was fully featured, however being able to storeRegToStackSlot did solve a major problem for me.

On the architecture I'm on (and many DSPs like it, I suspect), there's many special function registers that control various aspects of the CPU. These registers can be pushed, popped, and moved to/from GPRs. The issue is that the architecture has no "move from SFR to stack + offset" command, which LLVM requires for stack-frame store/loads. They must go through a GPR intermediate. Even solving the frame address would not avoid this issue, as there is no "move from SFR to [address]" either, they must either be directly pushed/popped from stack, or go through an intermediate.

Implementing "getLargestLegalSuperClass" to return a RegisterClass of (GPRs + SFRs) goes a long way towards avoiding these difficult spills, as they'll often be assigned a GPR over call-clobbering etc, but unfortunately AFAIK there's no way to tell the register allocator that spills must be from the GPR side of that union. Sometimes, it will request that the SFR gets spilled, and so in those instances you ultimately need a temporary in storeRegToStackSlot. One option would be a pseudo, followed by a pass using RegScavenger to provide the intermediate - but I would expect the codegen to be worse than generating the intermediate in storeRegToStackSlot, if supported.

Really though, this is all just a workaround to a problem. What's really desired/wanted, is just a way to say that SFRs must be spilled to GPRs, a "getSpillRegClass" if you will. This seems a missing piece/problem in general, whilst trying to find out how to address the problem I came across at least one question to llvm-dev on how to do it, unanswered iirc.

AMDGPU also seemingly has an extensive/complex workaround in "EnableSpillVGPRToAGPR", implemented in SILowerSGPRSpills, which if I understand it correctly, attempts to simplify similar two-stage spills post-regalloc by eliminating unnecessary frame indices. I'm not familiar with the architecture and may be wrong on the constraints they're working around, but on glancing it over, it seems to me that if a hypothetical "spillRegClass()" could be expressed, AMDGPU too would not need a temporary to be allocated in storeRegToStackSlot, and could also do away with much of SILowerSGPRSpills as well.

	/// Special case of eliminateFrameIndex. Returns true if the SGPR was spilled to
	/// a VGPR and the stack slot can be safely eliminated when all other users are
	/// handled.
	bool SIRegisterInfo::eliminateSGPRToVGPRSpillFrameIndex

Please do let me know if I've missed a better way at addressing this, perhaps there is just the function I'm looking for already.

Thank you,
Alex